1. Field of the Invention
The present invention relates to an image forming apparatus and a density correction method for correcting density of an image.
2. Description of the Related Art
Many types of image forming apparatuses form a full color image by superimposing a magenta image formed with a developer with a magenta hue, a yellow image formed with a developer with a yellow hue, a cyan image formed with a developer with a cyan hue and a black image formed with a developer with a black hue. A specific image forming apparatus corrects changes over time in density of developer. For example, the density is corrected using a toner patch formed on a photoconductive drum.
The specific image forming apparatus includes a transfer belt on which the toner patch for density correction is formed. FIG. 4 schematically shows the transfer belt of the specific image forming apparatus.
The transfer belt 1 shown in FIG. 4 includes a first edge portion 1a and a second edge portion 1b which extend in a moving direction of the transfer belt 1.
A test patch 2 for density correction is formed along the first edge portion 1a. The density correction using the test patch 2 is accomplished by a correction of a developing bias (bias calibration). A test patch 3 for density correction is formed along the second edge portion 1b. The density correction using the test patch 3 is accomplished by feeding an output density back to an input density to perform a γ correction (IO calibration).
The bias calibration and the IO calibration are performed using a density sensor 4 configured to detect density of the test patch 2 and a density sensor 5 configured to detect density of the test patch 3. As shown in FIG. 4, the density sensor 4 detects the density of the test patch 2 before the density sensor 5 detects the density of the test patch 3. A detection result from the density sensor 4 is fed back to the density detection of the test patch 3 by the density sensor 5. Thus, the detection accuracy of the test patch 3 by the density sensor 5 is improved.
Successive toner supply in a main scanning direction in the aforementioned image forming apparatus is likely to cause toner density variation between a first end edge of the transfer belt 1 where the toner supply starts and a second end edge of the transfer belt 1 where the toner supply ends, originated from variation in sensitivity of the photoconductive drum. Unless the density sensors detect any density variation in the main scanning direction, the density of the image largely deviates from target value, which in turn particularly leads to outstanding lower density.